The present invention relates to methods and arrangements in a user equipment (UE) and in a radio access network of a cellular mobile network. An example of such a radio access network is the UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System 100 connected to the Core Network (CN) 200. The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS 100 comprises at least one Radio Network Controller 110. Furthermore, the respective RNC 110 controls a plurality of Node-Bs 120,130 that are connected to the RNC by means of the Iub interface 140. Each Node B covers one or more cells and is arranged to serve the User Equipment (UE) 300 within said cell. Finally, the UE 300, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface 150. The network of FIG. 1 is also referred to as a WCDMA network and is based on the WCDMA standard specified by the 3:rd Generation Partnership Project (3GPP).
Requirements for mobile data access are increasing and demand for bandwidth is growing. To meet these needs the High Speed Data Packet Access (HSDPA) specification has been defined. HSDPA is based on WCDMA evolution standardized as part of 3GPP Release 5 WCDMA specifications. HSDPA is a packet-based data service in WCDMA downlink with data transmission peak rate up to 14.4 Mbps over a 5 MHz bandwidth. Thus HSDPA improves system capacity and increases user data rates in the downlink direction. The improved performance is based on adaptive modulation and coding, a fast scheduling function and fast retransmissions with soft combining and incremental redundancy. HSDPA utilizes a transport channel named the High Speed Downlink Shared Channel (HS-DSCH) that makes efficient use of valuable radio frequency resources and takes bursty packet data into account. This is a shared transport channel which means that resources, such as channelization codes, transmission power and infra structure hardware, is shared between several users. HS-DSCH supports HARQ as a fast and resource-efficient method for combating transmission errors.
In 3GPP Release 6, the WCDMA standard is further extended with the Enhanced Uplink concept by introducing the Enhanced Dedicated Transport Channel, E-DCH. A further description can be found in 3GPP TS 25.309 “FDD Enhanced Uplink; Overall description”. This concept introduces considerably higher peak data-rates in the WCDMA uplink. Features introduced with E-DCH include fast scheduling and fast Hybrid Automatic Repeat request (HARQ) with soft combining. Fast scheduling means that the Node B can indicate to each UE the rate the UE is allowed to transmit with. This can be done every TTI, i.e. fast. Thus, the network is able to control the interference in the system very well.
HARQ is a more advanced form of an ARQ retransmission scheme. In conventional ARQ schemes the receiver checks if a packet is received correctly. If it is not received correctly, the erroneous packet is discarded and a retransmission is requested. With HARQ the erroneous packet is not discarded. Instead the packet is kept and soft combined with the retransmission. That implies that even if neither the first transmission nor the retransmission would facilitate a successful decoding when received alone, they may be combined to decode the packet correctly. This means that, compared to conventional ARQ, fewer retransmissions are required.
For cellular systems that support full mobility, it is a particular challenge to support fast, seamless and lossless cell changes. This is true both for already deployed systems, e.g. GSM/GPRS, WCDMA, CDMA2000, and also for future systems, e.g. as depicted in 3GPP UTRAN Long Term Evolution (LTE) or 4G-systems.
In the 3GPP, work is ongoing concerning the evolution of UTRAN denoted as UTRAN long-term evolution (LTE). In LTE soft handover in uplink is considered as one way to increase the coverage and throughput. Soft handover, also denoted macro diversity, implies that the UE is connected to more than one base station simultaneously, and therefore transmits/receives the same information to/from the connected base stations. Soft handover, however, requires a central anchor node in the radio access network (RAN) where the packets from different radio base stations are combined. In LTE, it is possible that no central anchor node in RAN will exist from a user plane perspective (a central node in the control plane could still exist). In this scenario soft handover would not be supported. Instead, hard handovers could be used, i.e. data is only transmitted to one radio base station at a time. Such a hard handover is also referred to as cell change. To reach a high performance it is desirable to perform the cell change as fast as possible when a stronger cell is detected which could potentially lead to frequent cell changes.
In HSDPA and enhanced uplink the cell changes are performed independent of the ongoing HARQ retransmissions. This means that the outstanding data is lost at the cell change. In HSDPA/enhanced uplink these losses are recovered by the RLC layer, which provides retransmissions between RNC and UE. If no anchor node in RAN exists, the RLC layer, if at all present, cannot recover from these losses. Thus a solution is needed to provide fast lossless cell change, while still avoiding duplicates and maintaining in-sequence delivery. In WCDMA enhanced uplink in-sequence delivery is performed by reordering in the anchor node based on MAC level sequence numbers transmitted by the UE. This is not possible in a scenario without anchor node.
It is foreseen that a HARQ protocol is employed between the UE and the radio base station in LTE, similar to WCDMA enhanced uplink. This implies, if no special means are taken, that the data that is outstanding, i.e. has been transmitted but no ACK has been received, is lost at the cell change. This would mean that it is not possible to provide for a lossless cell change. Alternatively, the outstanding data could be retransmitted in the new radio base station, i.e. the radio base station after handover, which would avoid the data loss. This would however lead to unnecessary data transmissions and duplicated data in case the data was already received correctly in the old radio base station.
The HARQ scheme in LTE is expected to be similar to the scheme used in HSPDA and WCDMA enhanced uplink, i.e. an n-channel stop and wait protocol is used. Soft combining is applied, i.e. in case of retransmissions the retransmitted data is combined with previously stored information. N-channel stop and wait is the protocol used for HSDPA. It can be seen as N parallel stop and wait processes. A stop and wait protocol is a simple type of ARQ protocol where one PDU is sent and then the protocol waits (don't send any data) until an ACK/NACK is received. When the ACK/NACK is received either new data is transmitted or a retransmission is made. The efficiency of this protocols is low since the transmitter waits for the ACK/NACK to be received. In the N-channel stop and wait the Tx can send data in the other processes instead of waiting (but seen on each process its still stop and wait). Thus this protocol combines the simplicity of a stop and wait protocol with the efficiency of a window based ARQ protocol.
It is likely that the HARQ protocol by itself does not provide sufficient reliability due to the properties of the feedback signalling. Thus it is likely that LTE uses a separate layer of ARQ on top of the HARQ layer. In WCDMA this outer ARQ is located in the RLC layer. In LTE the outer ARQ may either be located in an RLC layer or be located in the MAC layer. Therefore, MAC-HARQ is referred to as inner ARQ and the ARQ in the existing RLC is referred to as outer ARQ.
In LTE, two types of Media Access (MAC) adaptation to the transmission media have been discussed, a radio centric MAC and a packet centric MAC.
The Radio Link Control Protocol (RLC) and MAC are layer 2 control protocols in the WCDMA radio access network and are likely to be present in LTE, although the functionality may be slightly modified. RLC in WCDMA provides transparent, acknowledged and unacknowledged services, and maps these services to logical channels. MAC in WCDMA is e.g. responsible for the mapping between the logical channels and the transport channels, for the control of E-DCH transmission including the support for HARQ and for the generation of uplink scheduling information to assist with E-DCH resource allocation. MAC and RLC are further described in TS 25.321 and TS 25.322. In LTE it is likely that a MAC layer is present which includes similar functionality as the WCDMA MAC layer. The RLC layer may be present in LTE but it is also possible that the RLC functionality is moved down to a new MAC layer in LTE.
In the radio centric MAC, which is most likely to be adopted, segmentation is applied either in MAC or in RLC. This means that in case a whole IP packet can not be transmitted in a transmission time interval (TTI) the IP packet is segmented into smaller protocol data units (PDUs) which are transmitted to the radio base station. Typically only a single PDU is transmitted per TTI. The IP packet can only be assembled in the radio base station in case all PDUs carrying segments of the IP packet have been received correctly. The radio base station transmits feedback information (ACK/NACK) for the received PDUs. Thus, when the UE has received positive feedback (ACK) for all PDUs carrying segments of an IP packet it is known that the IP packet is received completely. If the feedback for all PDUs has not yet been received, the UE does not know if the IP packet was received correctly or not but if at least one NACK is received, the UE knows that the IP packet cannot have been received completely.
In the packet centric MAC no segmentation is applied. Instead a complete IP packet is encoded and puncturing is used to reduce the number of bits (if needed) so it can be transmitted in one TTI. Additional redundancy is then transmitted in subsequent TTIs. To achieve a high efficiency the transmitter may transmit redundancy information for the same IP packet in several consecutive TTIs, until the likelihood that the IP packet can be decoded is sufficiently high. The UE then only needs to transmit one single ACK/NACK indication once all the redundancy versions have been received.